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New evidence about the age of ancient 'human' footprints in Mexico casts doubt on claims that humans really made them at all.

A new study, published in today's issue of the journal Nature, challenges a controversial claim made by an international team of researchers in July and suggests humans did not leave footprints in central Mexico some 40,000 years ago.

At that time, Dr Silvia Gonzalez, a geoarchaeologist at Liverpool John Moores University in the UK, and colleagues, announced the discovery of 269 fossil footprints, both animal and human.

The prints were found at the bottom of an abandoned quarry close to the Cerro Toluquilla volcano, southeast of Mexico City.

According to the researchers, the footprints were preserved as trace fossils in volcanic ash along what was the shoreline of an ancient volcanic lake.

About 60% of them were indisputably human, they said, ranging from children's prints to adults'.

It took Gonzalez's team nearly two years to date the material. The key date came from shells in sediments just above the layer of ash, which the team carbon-dated to 38,000 years ago.

The ages of sand grains baked into the ash confirmed the results.

The finding promised to shatter the conventional view that humans arrived in the Americas via Beringia around 11,000 years ago. The new evidence indicated that they had colonised the Americas 30,000 years earlier than previously thought.

The discovery has stirred controversy among scientists ever since it was made public.

At that time, Professor Paul Renne, a geochronologist at the University of California, Berkeley, said that the "eroded gouge marks" were unlikely to be human footprints.

In the current study, Renne reports a new dating of the volcanic rock carrying the impressions.

Using argon dating and palaeomagnetic analysis, Renne and colleagues examined nine samples of the volcanic rock.

"The alleged footprints are in the exact layer that I have dated, which is exposed over a large surface," says Renne.

He adds that the shells that provided Gonzalez's radio carbon dating are not embedded in that layer.

"The radiocarbon dates come from sediments overlying the volcanic material I have dated. Since the tuff is below the sediments containing the shells, it has to be older," Renne says.

All samples produced ages ranging from 1.26 to 1.47 million years, indicating that the basaltic tuff on which the purported footprints were found is in fact very much older than Gonzalez and colleagues claim.

At about 1.3 million years old, the footprints would predate the first known appearance of Homo sapiens in Africa by more than a million years.

Renne and colleagues called the possibility that the footprints were made by a hominid that existed before H. sapiens "extremely remote" and conclude that the impressions are not hominid footprints at all.

Evidence for earlier colonisation?

According to Gonzalez, the dates reported by Renne's group need to be replicated and independently confirmed.

"It is not clear from where exactly they took their samples and which fraction was dated. We took our samples directly from the footprint horizons," says Gonzalez.

"Even if we are wrong and the ash is indeed 1.3 million years old, that is not automatically a reason to disregard interpretation of the features reported as footprints, simply because they are not in agreement with the established models for the settlement of the Americas," she says.

Renne's study has raised serious doubts among scientists.

"The new dating is far beyond any credible evidence of humans in the Americas. Some experts had questioned whether the prints were indeed human, and this issue will now have to be reexamined very carefully," says Professor Chris Stringer, head of human origins at the Natural History Museum in London.